High solidity vanes in centrifugal compressor diffusers have high efficiency but poor range. Vaneless diffusers have wide ranges but poor efficiency. Low solidity vanes are known as a useful compromise between high solidity vanes and no vanes. High solidity is defined as a vane chord to pitch ratio greater than one; low solidity as a chord to pitch ratio less than one. Low solidity vane diffusers are somewhat efficient and have nearly as wide a range as vaneless diffusers. It has also been shown that tandem low solidity vanes, acting like split airfoils, result in even higher efficiency than ordinary low solidity vanes, yet have the same range. However, low solidity vanes are generally unable to yield the efficiencies associate with high solidity diffuser vanes, because of the lack of positive guidance in the air flow in the usual low solidity design.
The standard practice in the centrifugal compressor art is to provide an impeller with between 15 and 19 blades. For good efficiency it is desirable to provide 10-50 percent ore diffuser vanes than impeller vanes. This is why many prior art diffusers have between 19 and 22 vanes.
A significant improvement to the art was contributed by Dr. Senoo, who proposed leaving the number of vanes essentially the same, but decreasing the solidity or chord to pitch ratio to less than one. Further, improvements were realized with the tandem vanes, low solidity design disclosed by Senoo et al. in ASME publication 83-GT-3. In doing so, Senoo chose to decrease the number of vanes from about 22 to 11, to maintain low solidity.